Abstract: The frequency of African American and Latina women diagnosed with HER2-overexpressing breast cancer is higher compared to Caucasians and they are at a higher risk to die from breast cancer. Trastuzumab (Herceptin) has been shown to successfully treat HER2-overexpressing metastatic breast cancer and prolong the survival of patients with HER2-overexpressing breast cancer. However, more than 50% of patients with HER2-overexpressing breast cancer will eventually become resistant to trastuzumab. Preliminary data from our laboratory suggested that up-regulation of Wnt3 (proto-oncogene; ligand for members of the frizzled family) may play a key role toward the development of trastuzumab resistance and metastatic disease in HER2-overexpressing breast cancer. It is interesting to know if inhibition of Wnt signaling could restore the sensitive of trastuzumab and prevent metastases of HER2-overexpressing breast cancer. We hypothesize that Wnt3 associated network of signaling pathways could drive HER2 overexpressing breast tumors to acquire resistance to trastuzumab and increase tumor metastasis. The increased expression of Wnt3 will activate Wnt/?-catenin signaling, promote HER2 cells toward Epithelial to Mesenchymal Transition (EMT), and initiate tumor metastases. Hence, targeting Wnt3 may enhance or restore the HER2 overexpressing breast tumor's response to trastuzumab treatment and consequently inhibit metastasis. To test our hypothesis, we propose three specific aims in this study: (1) Examine the expression of Wnt3 in relation to trastuzumab resistance and tumor metastases in cohort of African American and Latina patients, and the association of Wnt3 with HER family receptors and molecular subtype of breast cancer. We will compare the data obtained from the cohort of African American and Latina with a cohort of samples from Caucasian; (2) Examine the mechanism(s) by which the Wnt3?Wnt/?-catenin pathway initiates and sustains resistance to trastuzumab in HER2-overexpressing breast tumors, and identify the?driver?or ?keymediators? that determine how the HER2-overexprerssing epithelial cells transition to more mesenchymal phenotype and eventually to metastasis; (3) Test inhibition of Wnt3/Wnt ligand in combination with trastuzumab and/or Lapatinib to overcome HER2-cells resistance to trastuzumab and prevent metastases in vitro and in vivo. Activity from the proposed project will provide research education opportunity for students and fellows.